The photoreceptors of the teleost retina elongate and contract in response to light and dark. This project studies the underlying mechanism responsible for this cell shape change. Previous studies have shown that microtubules are required for elongation and that they undergo characteristic changes in distribution during elongation. Also we have shown that the retinal cone has paraxially aligned thin (actin) and thick (probably myosin) filaments. Proposed studies attempt to develop a motile model of isolated, detergent treated cones which is capable of contraction on addition of calcium and ATP. This model would be used to characterize ionic, nucleotide, pH requirements for contraction. EM studies of extracted contractile cells would attempt to characterize the minimal machinery necessary for contraction. Morphological studies using Heuser's quick-freeze, deep etch fracture technique will attempt to capture associations between tubules or filaments during elongation or contraction.